Thanks for your offer ! I actually live in Canada. I don't think I need to be that precise about dimming. About dissipation, consider I need to turn on my instrument maximum 15 times a week x 5 mins. If the rheostat your proposing me can theoretically give me the full 10 watts and dim a bit without overheating during that time, I don't really care about paying 40$ on a guess. I almost do it for fun and I'm pretty sure I can order such a "connecting box" from the distributor of the ophtalmoscope. I think I'll be able to get that kind of a rheostat in Canada. I'll order soon and give you my observations...